Coulure (Verrieselung)

Definition

Coulure (German: Verrieselung; French: coulure) is a physiological problem in viticulture in which the fertilisation of flowers is incomplete or the young berries fall off after flowering. This results in loose, open clusters with significantly fewer berries than normal — yields drop considerably.

Causes & Formation

Coulure occurs primarily during the critical flowering period, which usually takes place from late May to early June. Several factors can trigger the phenomenon:

Adverse weather conditions during flowering are the main cause: cold, wet, or windy weather hinders fertilisation. At temperatures below 15°C and with prolonged rainfall, pollen dispersal is disrupted and flowers cannot develop fully.

Genetic predisposition plays an important role. Some grape varieties are more prone to coulure than others. Particularly susceptible are Pinot family varieties such as Pinot Noir (Spätburgunder), Pinot Meunier (Frühburgunder), and Gewürztraminer. Merlot also shows increased susceptibility.

Nutrient deficiencies, particularly of boron and zinc, can impair fertilisation. A disrupted nutrient balance during the sensitive flowering period promotes the dropping of young berries.

Excessive vegetative growth causes the vine to invest its energy in shoots and leaves rather than grape formation. This is often caused by nitrogen-rich soils or excessive fertilisation.

Effects

On Yield

Yield losses from coulure can be significant — in severe cases up to 50–70% of the normal harvest quantity. For growers, this means considerable economic losses.

On Quality

Paradoxically, coulure can improve wine quality. Loose clusters have several advantages:

• Better ventilation of the clusters reduces the risk of fungal diseases such as botrytis
• More concentrated berries through natural yield reduction leads to more intense aromas
• More favourable ratio of skin to juice increases the concentration of colour compounds, tannins, and aromatic substances

Many top winemakers therefore regard moderate coulure not as a disadvantage but as natural quality selection by nature.

Affected Grape Varieties

Particularly susceptible to coulure are:

• Pinot family: Pinot Noir, Pinot Meunier, Pinot Gris, Pinot Blanc
• Gewürztraminer: One of the most susceptible varieties of all
• Merlot: Especially in cooler climates
• Grenache/Garnacha: In unfavourable weather
• Muscat varieties: Gelber Muskateller, Muscat Blanc

Less susceptible are more robust varieties such as Riesling, Cabernet Sauvignon, or Syrah.

Prevention & Management

Growers can reduce the risk of coulure through various measures:

• Targeted canopy management in spring to control excessive vegetative growth
• Balanced nutrient supply with particular attention to boron and zinc
• Site selection: Prefer sheltered, warm sites for susceptible varieties
• Moderate irrigation and fertilisation to avoid excessive growth
• Clone selection: Use of clones with lower coulure tendency

Connection to Quality Winemaking

In modern quality winemaking, coulure is increasingly viewed with nuance. While any yield reduction was once considered a problem, many top winemakers today recognise the quality benefit of naturally coulure-affected grapes. Some speak of "accepted" or "tolerated" coulure as a natural means of yield reduction — without manual bunch thinning.

The best examples are found in Burgundy, where coulure-affected Pinot Noir grapes often produce the most complex and long-lived wines. Along the German Ahr, growers of Pinot Meunier (Frühburgunder) also value the natural concentration achieved through coulure.